def delete(y):
    flag = 0
    for d in range(2, y):
        if y%d == 0:
            flag = 1
            break
    if flag == 0:
        return y
    else:
        return 0
x=int(input(""))
if x < 10000 and x > 999:
    y=str(x)
    a=int(y[0]+y[1])
    b=int(y[2]+y[3])
    c=int(y[1]+y[2])
    d=int(y[0]+y[1]+y[2])
    e=int(y[1]+y[2]+y[3])
    f=int(y[0])
    g=int(y[1])
    h=int(y[2])
    i=int(y[3])
    j=int(y[0]+y[1]+y[2]+y[3])
    z=[]
    z.append(delete(a))
    z.append(delete(b))
    z.append(delete(c))
    z.append(delete(d))
    z.append(delete(e))
    z.append(delete(f))
    z.append(delete(g))
    z.append(delete(h))
    z.append(delete(i))
    z.append(delete(j))
    print(max(z))
else:
    print("0")
